Bernoulli's Principle and Airflow

[flyinguitar]

Actually, that's an excellent example of a thought experiment that shortcuts around a lot of calculations.

Gedankenexperiment! Do you have a physics background too?

Not really. Pressure itself is Newton's third law in action; if there is pressure against the wing, we know that momentum has been transferred to the air and Newton's third law is satisfied. There is no need for the coherent flow of the air be deflected downwards.

And downwash isn't merely "downward-rotating part of a wingtip vortex", it's the net deflection of the entire relative wind. According to theory, this does not happen in the absence of wingtip vortices, but it does not consist solely of vortices.

However, even in the absence of vortices and absence of downwash, there is still a pressure difference between the top and bottom of the airfoil, which means there is a reciprocal momentum transfer between the air and airfoil.

I need to think and research before commenting more. But one thought: does the following SAT analogy have any truth?

Tipless wing in wind tunnel is to finite airplane wing as is ducted fan to free propeller.

If it is, it has some implications, since we all know turbofans move a LOT of air.

 

[tgrayson]

But one thought: does the following SAT analogy have any truth? Tipless wing in wind tunnel is to finite airplane wing as is ducted fan to free propeller. If it is, it has some implications, since we all know turbofans move a LOT of air.

It's certainly a question worth exploring. My comment about the no net air deflection in a tipless environment is from statements in aerodynamics books to that effect. I can't defend it beyond that point.

Is the duct fan equivalent? I think there are some differences. In a duct fan, the aircraft is moving perpendicular to the direction of motion of the airfoil. The main wing, however, is moving parallel to the motion of the aircraft.

So the duct fan basically sits motionless in the column of air that it's acting on and the low pressure in front of the fan causes a steady acceleration of the air ahead of the inlet. The main wing, however, is only in a column of air very briefly and the resulting acceleration of the air would be minute.

 

[fish314]

Only briefly and minutely. The static longitudinal stability will ensure that V will change in response to CL changes. There's only one real independent variable in the lift equation and that's CL.

I don't follow how you come up with that, T., unless you are assuming constant thrust? Or constant lift?

Because, as I see it, I can achieve a desired AOA (as measured by my AOA guage) in my aircraft, and hold it at a desired airspeed, provided I don't care about other quantities, like altitude or bank angle, etc..

The G loading for a given AOA will be different, depending on the airspeed (and so will the power setting required to acheive it), but I think that I can set them independently.

The example I have in mind are Maximum g level turns, at 2 different airspeeds (for example 120 knots, and 180 knots). As an example, let's say maximum G at 120 knots for a particular aircraft is 2 at 120, and about 4.5 at 180.

Since we're talking about maximum G turns we've set the AOA just below critical AOA for both turns. The AOA guage and the presence of all of our pre-stall indications (buffet, stall horn if so equiped, stick shaker, etc.) confirm we are near stall AOA. But we've also set airspeed.

So we've set them independently. And since we're taking CsubL as basically a function of AoA, we've set CsubL independent of V, haven't we? What am I missing?

 

[tgrayson]

unless you are assuming...constant lift?

Yes, I'm assuming constant lift. If you add thrust, the initial tendency is for the aircraft to accelerate. However, with a constant AOA, you will have lift > weight, which will cause the flight path to arc upwards. As a component of gravity starts to tug on the aircraft in the drag direction, the aircraft will slow down to its original velocity and lift will once again = weight. Therefore, what you have as a result of the increased thrust is a climb, rather than an increase in velocity. That's why you can't control V independently of CL.

The G loading for a given AOA will be different, depending on the airspeed (and so will the power setting required to acheive it), but I think that I can set them independently.

Agreed, if you're talking about changing your g loading, you can set AOA and V independently within a range.

What am I missing?

Nothing, it appears. However, reading your original post, I didn't gather you were referring to this context and I think the casual reader (or listener) might think you were saying what I thought you were saying and miss the dependence of airspeed on AOA. Looking back, it appears you may have just been describing what produces lift *in general*, without respect to how we actually do it in the airplane. In a wind tunnel, for instance, you can control all the variables.

I find that I have to be very careful when discussing the lift equation with students, because they will often become overly concerned with generating lift in the airplane. That sort of makes me back up and stress that they don't have to worry about generating lift, the airplane will always generate enough lift to support the weight of the airplane; the only thing we can control is the speed and altitude at which that happens, using the yoke and throttle respectively. So when I write the lift equation, I will quickly cross out the "L" and replace it with "W" or "nW".

Another misinterpretation some draw is the role of air density, probably based on the Commercial Oral Exam Guide. The reader walks away with the idea that a decrease in density will increase the stall speed. It *does* increase the TAS of the stall, but not the IAS. Some people pack on unnecessary airspeed to compensate on a hot day.

 

[fish314]

ah, well, cool. Thought I was going nuts there for a second!

Yeah, if Lift=Weight (like in most flight applications), then there is only one variable. To me it seems to make more sense to think of it as velocity, than as CsubL, but I guess that because they are intertwined anyway it's kind of the same.

I wonder if the way we're looking at it has to do with where we each stand on the "pitch for airspeed, power for glideslope vs. pitch for glideslope, power for airspeed" debate?

 

[tgrayson]

To me it seems to make more sense to think of it as velocity, than as CsubL, but I guess that because they are intertwined anyway it's kind of the same.

Yes, except there is no flight control that changes V directly. ---Unless, using your example, you want to say "aileron". ---

 

[RynoB]

The latest Flight Training magazine had an article on lift. The first thing discussed was how air flowing over the top of the wing must flow faster to meet the air flowing under the wing at the trailing edge. I couldn't make myself read any further.